1. Field of the Invention
This invention relates to an illumination system for a film scanner and more particularly, to an aperture control system suitable for wide dynamic range of operation to a specified illumination level within the time allotted for scanning a film frame in real time in a telecine film scanner.
2. Description of the Prior Art
State of the art illumination systems for telecine film scanners typically include light-sensitive, charge-coupled device (CCD) linear arrays which provide a serial output representing a line of a television raster and a light source for illuminating a linear section of the film being scanned with either a scanning light beam or a line of diffuse light. For color television, one version of a film scanner can include a light source that is subjected to dichroic filtration to tailor the spectral content of the light according to the film to be scanned and directed into a light integrator which transforms a light beam from a light source into a line of diffuse light emitted through an elongated aperture for illumination of the film frame. The line of light must be sufficiently uniform in intensity and diffusion along its length and have the red, green and blue spectral radiance for the film being scanned. The film is held in a cylindrical shape and moved past the line of diffuse light. The light passing through the illuminated linear section is color and intensity modulated by the image therein and imaged onto three color filtered CCD arrays as described, for example, in commonly assigned U.S. Pat. Nos. 5,012,346 to DeJager et al. and 5,003,379 to Moore, Jr. et al.
The film motion provides the vertical (frame) scan, and the linear, synchronized electronic cycling of the CCD arrays provides the horizontal (line) scan of a rasterized television signal with both chrominance and luminance components. A number of the considerations taken into account in scanning positive or print and negative color film stocks with such telecine film scanners is set forth in the article "Optical scanning system for a CCD telecine for HDTV" by Kurtz et al. in SPIE, vol. 1448 Camera and Input Scanner Systems, 1991, pp. 191-205.
As described in that article and in commonly assigned U.S. Pat. Nos. 4,868,383 and 5,155,596, to Kurtz et al, and 5,012,346, to DeJager et al, such film scanner linear light sources have difficulty transforming light emitted by a source lamp into a line of diffuse light of sufficient, uniform intensity or brightness for the full lamp life and operating intensity range required and which does not vary from moment to moment due to lamp "noise". In the '383 and '346 patents, a linear light integrator is provided with a light intensity sensor and feedback circuit for regulating the lamp power supply to dampen light output intensity fluctuations of the Xenon arc lamp. Intensity fluctuations from the targeted lamp intensity may be both temporal and spatial, as arc length and position within the lamp envelope varies and shadows caused thereby change from moment to moment. The severity of the fluctuations increases as lamp current is decreased, so the Xenon arc lamp is driven at a relatively high current and intensity.
The integrating cylinder is designed to integrate the light beam from the lamp so that a line of light is emitted that is uniform in intensity along the length of the aperture at any given moment, regardless of spatial fluctuations in light beam intensity. Consequently, the intensity of the uniform line of light varies temporally. The temporal variation in integrated light intensity from the target or reference intensity is referred to as illumination "noise" that is considered in measuring the illumination signal-to-noise (S/N) ratio performance of the system. In the '596 patent, the instantaneous intensity of the light in the integrating cylinder is monitored by the light intensity sensor and feedback circuit to develop a correction factor to increase or decrease the lamp drive current to improve the overall illumination S/N performance.
As described in the '596 patent, there is also a need to regulate the intensity of light entering a light integrating cylinder in response to changing characteristics of the optical path, the film scanning speed, the film type, and the ageing of the arc lamp. Light intensity can only be varied in a narrow range by controlling current to the short arc Xenon lamp, because lamp life is shortened as lamp current is increased, and illumination S/N performance is degraded as lamp current is decreased. Moreover, when CCD arrays are employed, the further electrical signal-to-noise (S/N) performance of the CCD elements as a function of light intensity limits the range of variation in illumination.
The CCD elements, in converting light into electrical output signals, create a relatively fixed level of electrical operating noise which is included with the output signal representative of the intensity of the light transmitted by the image pixels. If the elements are not charged by the pixel illumination intensity to a level in the normal intensity variation operating range, e.g. may occur when low intensity light is blocked in a dark scene, the fixed noise contributes a greater proportion to the output signal, and the S/N ratio decreases, leading to loss of image detail. Widening the lamp intensity output range would therefore degrade system S/N performance in the low output regions of the range. Lamp current control is thus relegated to a narrow range of modulation of the lamp current in response to illumination level S/N variations in lamp arc performance.
The wide dynamic range of adjustment necessary to scan different types of film is left to a motor driven aperture wheel described in the '596 patent and the above-referenced article. The aperture wheel intercepts the light beam from the arc lamp before it enters the integrating cylinder and is set to provide a specific aperture to adjust overall light intensity in the light beam depending on the film type and operating conditions of the lamp for the duration of scanning of the film.
There are additional reasons for adjusting the intensity of the light beam than compensating for film type and for temporal fluctuations in lamp output. Typically, a given movie contains a variety of scenes resulting in differing average film densities from scene to scene, e.g. dark night and bright day scenes. In a darkened theater, when the scenes are projected from the print movie film, even the darkest scenes are visible, and the contrast between the darkest and lightest parts of the scene is high. However, the television viewing experience is quite different. The television is usually viewed in moderately to brightly lit surroundings and this ambient light reduces the contrast between dark and bright portions of the image. Furthermore, the television screen image contrast and resolution is limited by phosphor flare and lag resulting in a lower resolution and dynamic contrast range than a projected print movie film. The combined effect of the bright ambient lighting and the low dynamic range screen is that television images of a movie have considerably less visual contrast when compared to the original projected print.
To compensate, the experienced telecine operator or colorist will pre-program the color balance and signal level settings on a scene by scene basis. The colorist seeks to compensate for the deficiencies in television images and to still provide artistic qualities of each scene similar to the original projected scene.
For a dark scene, where even the highlights are dark, the colorist may increase the signal level such that the highlight is brought to 100% video signal level. This also has the effect of increasing the average signal level for the entire scene. Though this gain in signal level could be adjusted electronically, doing so has the effect of increasing noise levels as well. The preferred method for increasing the video signal would be to increase the illumination level by increasing the amount of light emitted by the integrating cylinder and directed on the film image frames. However, current telecine systems are incapable of changing light intensity at real time film frame rates.
Summarizing the above, the telecine conversion of negative, master and positive, print movie films for television broadcast or reproduction on videotape or videodisc for playback on television, particularly HDTV, involves a number of considerations described amply in the cited references. Temporal fluctuations in the arc lamp output intensity is one of the significant causes of flicker in the resulting video playback image. In addition, for aesthetic reasons relating to the differing viewing conditions and the limited or differing dynamic brightness range of television screens, it is desired to alter the scanning illumination upon a scene change from the baseline commanded light intensity value. The baseline commanded light intensity value is defined depending on the film type and has been set in the system described in the '596 patent by the position of the aperture wheel for the duration of the telecine conversion.